A Currentless Circulating Current Suppressing Control for Modular Multilevel Converters

Circulating current harmonics of the modular multilevel converter (MMC) are superimposed on arm currents, increasing the RMS values of arm currents and increasing losses. It is necessary for the harmonics of circulating currents to be suppressed. Most existing methods of circulating current suppression require sensing of the arm currents, which increases sensors and sampling circuits, increases cost, and decreases the reliability. This paper proposes a currentless circulating current suppressing control through introducing proportional-resonant controller to suppress the harmonic voltages, thereby suppressing the harmonic currents, where the harmonic voltages are obtained by the calculation of duty ratio for submodules (SMs) and capacitor voltages in each phase. The proposed control can be easily executed by the Digital Signal Processor (DSP) controller without accessing the switching signals which are obtained in the Field Programmable Gate Array (FPGA), and it is suitable for single-phase or multiphase systems. In addition, it does not require the acquisition of arm currents. Simulation studies are implemented, the effectiveness is verified.